Partitioning of active agents between polar and nonpolar phases has a key role in the early stage of drug and drug-carrier design in the pharmaceuticals industry, as well as for separation of products in biosynthesis. In the present paper, the group-contribution Universal Quasi-Chemical Functional-Group Activity Coefficient (UNIFAC) and the a priori Conductor-like Screening Model for Real Solvents (COSMO-RS) models are applied to predict micelle/water partition coefficients. The models allow predictions based only on the molecular structure. The practical implementation of the models is examined by studying several homologous series of organic solutes in aqueous solutions of non-ionic (polyethoxy alcohols) and ionic surfactants (sodium dodecyl sulfate (SDS)). Good quantitative agreement with experimental data from the literature has been achieved. Factors that seem to be important in the calculation and to influence the prediction results are discussed. Among these are interfacial contribution and conformation analysis. Compared to UNIFAC, the COSMO-RS method opens up new perspectives, because ionic components, steric isomers, and inorganics can be modeled.